How to Properly Use the Objective Lenses and Focusing System of a Microscope
When using a microscope, follow the focusing principle: low power first, then high power. Focusing under the low‑power objective serves as preliminary focusing for the high‑power objective. When switching to the high‑power objective, simply rotate the nosepiece. The focal distance preset with the low‑power objective remains unchanged. Under high power, only a slight fine adjustment, or even no adjustment, is needed to observe the microstructure. However, many operation manuals do not clearly specify what the "low‑power objective" refers to.
In microscope operation, the 10× objective is the standard and commonly used lens for focusing. The reason is that switching from the 10× objective to either a lower or higher magnification objective does not bring abrupt changes. Another reason is that objectives of even lower magnification have a large depth of focus. It is difficult for the human eye to obtain an accurate focus, and collision between the sample and lens is likely when switching directly to high power afterwards.
Meanwhile, the 10× objective is widely used in practical work. For example, many national standards for metallographic inspection frequently reference standard charts under 100× observation. This 100× magnification is achieved by combining a 10× objective with a 10× eyepiece. From a practical operation perspective, provided the previous operations are proper, the objective will stay near the focal plane. After the specimen is correctly positioned under the 10× objective, a blurry or fairly clear image will appear, and only minor fine focusing is required.
(2) Differences in practical experience Our practical experience regarding focusing after switching from low‑power to high‑power objectives differs greatly from descriptions in other references. Thanks to improved manufacturing techniques, modern microscopes, especially imported models, feature good parfocality among objectives. Once focused clearly under low power, the image may remain sharp after switching to high power without refocusing. If adjustment is needed, only a tiny increase in object distance is required. The adjustment is not measured by 1–3 turns, but merely 1–3 degrees of rotation - an extremely small amount.
(3) About the objective nosepiece Do not push the objective lens directly by hand when switching objectives. This may loosen the objective thread and tilt the optical axis. Microscope objectives and digital imaging adapters are tightened onto the nosepiece. To change objectives, rotate the nosepiece until a faint "click" sound is heard and obvious resistance is felt. At this position, the objective is properly seated, perpendicular to the stage surface.
